The principle structure of a medium or high voltage cable with an envelope of plastic material is such that the conductor which usually is consisting of a plurality of wires is encircled by a field controlling or field smoothing layer. This layer is encircled by an insulation layer of plastic material of more or less thickness. The insulation layer is encircled by an outer semiconductor layer. The semiconductor layers are relatively thin if compared with the insulation layer and are also of plastic material. For conducting purposes these layers are added with graphite in order to achieve a weak conducting property. The purpose of these semiconductor layers is to influence the electric field strength within the insulation layer.
The envelope consisting of the individual layers is usually applied to the conductor by an extrusion process by means of a suitable extrusion head. A cross-linking path joins to the extruder wherein the plastic material is cross-linked. A plurality of inspection and controlling devices are known in order to inspect the desired properties and qualities of a cable. An important criterion is the diameter of a cable. A plurality of proposals have been made for measuring the diameter which normally base on optical measuring methods. A further criterion is the position of the conductor within the envelope. Eccentricities result in a reduction of the thickness of the insulation to one side and therefore include the danger of a breakthrough. In DE 44 12 122 C1 a method for measuring the eccentricity of a conductor within an insulation of a cable has been disclosed. Furthermore, it is known to measure the wall thickness of the cable insulation. In this method the difference between the outer diameter of the conductor and the outer diameter of the insulation is measured.
In order to measure the wall thickness and also the diameter, in particular for an envelope consisting of a plurality of layers it has become known to direct x-rays to the cable and to project the shadows on a x-ray receiver. This known methods also allow to determine the thickness of the inner field smoothing layer and of the outer semiconductor layer. It has been also become known to provide two measuring paths orthogonal to each other (XY-axes).
The described manufacture for high voltage cables does not exclude that interferences or interruptions for the inner field smoothing layer occur. In this case the field strength in the insulation is increased at the interruptions so that a breakthrough may occur. By means of the described x-ray method short interruptions of the field smoothing layer cannot be detected, in particular if they are beyond the measuring paths.
From EP 0 394 525 A1 it has become known to move a cable through at least two annular probes arranged one after the other. The probes each form capacitances with the conductor and are loaded by a voltage source such that upon failures in the insulation the capacity is short circuited by a breakthrough current. This method allows to detect failures of the insulation and also of their position thereof. Disturbances or interruptions in the inner field smoothing layer cannot be detected by the known method. The contribution of this layer to the total insulation for the cable sheath is neglectable. In the known method, no breakthrough would occur if the outer semiconductor layer or the inner field controlling layer is interrupted in a certain area.
Apparatuses or methods which allow by simple means to reliably detect interruptions or interferences of the mentioned layers for medium and high voltage cables are not known.
For cables which serve for the transmission of signals or for low voltage applications, it has become known by GB 2 003 613 A to measure the specific capacitance. The cable which normally has only one insulation layer is moved through a measuring tube filled with water. A high frequent measuring voltage is connected to the tube. The current which flows between the measuring tube and the conductor is depending upon the dielectricity of the insulation material as well as upon the thickness in connection with the outer diameter. For the manufacture of such cables, the maintenance of the predetermined specific capacitance (capacitance per length unit) is important since the magnitude of the capacitance in conjunction with the inductance defines the characteristic impedance of the cable. The user of the cable must consider a specific characteristic impedance. From DE 198 09 890 C1 it has become known to provide in the cooling path of an extrusion plant a first measuring electrode for the measuring of the specific capacitance and a second measuring electrode having a smaller length than the first measuring electrode. The current between the first and the second electrode on one side and the conductor on the other side is measured. The second measuring electrode has the purpose to measure changes of a capacitance occurring temporarily. If such temporary capacitance changes occur periodically, this results in reflections for the data transmission and high frequency cables and thus in losses at the discrete transmission frequencies. Thus, a so-called “structural return loss” occurs which apparently should be as small as necessary.
In connection with medium and high voltage cables, the capacitance is not measured since the knowledge thereof is not necessary for the operation of the cables.
The invention relies on the problem to provide a device by which in a simple manner interferences and interruptions of the inner field controlling and/or the outer semiconductor layer for medium and high voltage cables can be detected.